This invention relates in general to reel to reel web transport apparatus and more particularly to a motion control system for reel to reel web transport apparatus which maintains a controlled tension on the web at all times, even during loss of power to the web transport motors.
Reel to reel web transport apparatus are used to transport a web between reels past a utilization device. For example, magnetic tape recorders are widely used to record and reproduce, audio and video signals, instrumentation signals and digital data. In the transport of magnetic tape between reels, past record/reproduce (read/write) heads, the tape is subjected to velocity variations (such as acceleration, constant velocity and deceleration) which cause tape tension variations. In order to effect interchangeability of tapes between different tape recorders and to minimize degradation in recording and reproducing of information on the magnetic tape, it is desirable to maintain a controlled tension on the tape at all times. If the tape tension is too little during tape transport, the tape may slacken and become damaged by entanglement in the tape transport mechanism or by loose winding on the takeup reel. If the tape tension is too great during tape transport, the tape may become stretched during recording or reproducing, thus causing undesirable signal distortion and information degradation. It is thus desirable that a controlled tension be maintained on the tape at all times including loss of power to the tape drive motors.
Since the inertia of a reel of magnetic tape varies as the diameter of the tape pack decreases or increases during supply and takeup, tape tension also varies. A tape buffer or storage device has been used in order to minimize the effect of tape tension variations. Buffer devices, such as spring loaded storage arm devices and vacuum column buffer devices, are disadvantageous because of increased mechanical, electrical and control complexity and increased space requirements. Capstan drive tape transports are also disadvantageous due to mechanical and electrical complexity.
It has been proposed in U.S. Pat. No. 3,806,061, entitled "Tape Transport", issued Apr. 23, 1974, and U.S. Pat. No. 3,809,061, entitled "Tape Transport", issued May 7, 1974, to provide a simpler reel-to-reel tape transport by using a single reversible motor and constant force magnetic couplings to provide drag to the reels via one-way-clutches. The transports disclosed in these patents are disadvantageous because the single motor drive, although simple, lacks the flexibility of individual motor control for each reel to adapt to varying conditions of tape transport.
Thus, a reel-to-reel tape transport which uses separate motors for driving each reel offers several advantages over capstan drive transports and reel-to-reel transports, with or without buffer devices, for tape tension and speed control. The bufferless and capstanless reel-to-reel tape transport is simpler in mechanical structure, more space efficient, and requires fewer mechanical components. It also allows for a simpler and more efficient control system since only the reel motors are controlled. Thus, a motion control system for a reel-to-reel web transport apparatus should effect controlled tension of the web at all times over the full range of tape motion (e.g., at rest, acceleration, constant speed, and deceleration) in order to minimize signal degradation caused by undertensioning or overtensioning the web. Advantageously, in the event of loss of power to the reel drive motors or loss of intelligence to the control system, the motion control system should bring the web to a gentle and controlled stop.